This commit enforces the requirement of Content-Type for the REST layer and removes the deprecated methods in transport
requests and their usages.
While doing this, it turns out that there are many places where *Entity classes are used from the apache http client
libraries and many of these usages did not specify the content type. The methods that do not specify a content type
explicitly have been added to forbidden apis to prevent more of these from entering our code base.
Relates #19388
The RestHighLevelClient class takes as as an argument a low level client instance RestClient. The first method added is ping, which returns true if the call to HEAD / went ok and false if an IOException was thrown. Any other exception gets bubbled up.
There are two kinds of tests, a unit test (RestHighLevelClientTests) that verifies the interaction between high level and low level client, and an integration test (MainActionIT) which relies on an externally started es cluster to send requests to.
Luca Cavanna
Jim Ferenczi
Jay Modi
javanna